Various proposals have been made in the art for achieving effective marking of substrate, by causing a change of color in or on the substrate, on which the marking is to appear. In the art, various irreversible thermochromic marking components, i.e. activatable pigments, are disclosed. Substrates coated with such irreversible thermochromic marking components may be colored by application of heat, such as by lasers; thereby the coated substrate may be marked. Marking of a substrate with a laser or some other irradiation source, may be also denoted printing or imaging, as the laser and/or the substrate typically is moved during the process of marking the substrate.
As an example, WO 02/01250 discloses the use of oxymetal salts, such as ammonium octamolybdate (AOM), as marking component in laser marking. AOM is an example of a marking component that may be marked directly with 10,600 nm laser radiation. Laser radiation having a wavelength of about 10 μm may for example be obtained by use of mid-IR CO2 lasers with an emission wavelength in the range 10,000 nm to 12,000 nm. However, mid-IR CO2 lasers are less suitable for installation into existing production lines due to their physical bulk size.
An NIR fiber laser may have a small print-head fitted to the production line, connected to the laser, several meters away, via an umbilical cord. Thus, the disadvantage or mid-IR CO2 lasers, being less suitable for installation into existing production lines due to their physical bulk size, may be overcome by use of a NIR (near infra-red) laser and by addition of a NIR-absorber to the ink formulation. The NIR-absorber will absorb near infra-red laser irradiation and convert it into conductive heat. Thus, ink formulations comprising a marking component, e.g. AOM, and a NIR-absorber may be marked by use of a NIR-laser instead of a mid-IR laser. A common example of a NIR-laser is Nd:YAG laser.
Leuco dyes are marking components that are colorless in one form, but may change into a colored form upon exposure to a particular stimulus. Most leuco dyes are halochromic, i.e. they respond to changes in pH, typically being colorless under alkaline conditions, but being colored in an acidic environment. The color change of halochromic leuco dye may occur when the leuco dye is protonated. Such protonation may result in changes in the conjugate systems of the leuco dye, thereby a conjugated system may be form, having ability to absorb photons of visible light, and therefore appearing colored.
By combining a halochromic leuco dye and a thermal acid generator (TAG) releasing protons upon heating, also halochromic leuco dye may be used in thermal marking, such as in laser imaging. A well known and effective TAG is benzylhydroxybenzoate, which in combination with various halochromic leuco dyes, provides ink formulation having good markability.
In certain applications there is a need for covering the substrate with a thermoplastic polymer layer. By covering the ink substrate with a thermoplastic polymer layer, certain properties of the substrate, such as the impermeability to liquids are improved. Further the thermoplastic polymer layer also protects the marking och the image. For packaging material, typically being a laminate including at least one layer of paper, cardboard, corrugated paper board, or paper board, the outermost layer, or one of the outer layers covering, the substrate is typically low-density polyethylene (LDPE)) or polypropylene melt extruded onto the laminate. For such polyolefins, a temperature of 200 to 340° C. is typically used in melt extruding the polyolefin.
As already described, marking components in the art are typically thermo activated. Typically, discoloration is thus seen, due to activation of the marking component, if such marking components are over laminated by melt extrusion. Accordingly, heat activated marking components are thus presently coated on top of the outermost layer of packaging material laminate, as said layer typically is a melt extruded polyolefin layer, in order to avoid discoloration prior to marking.
In order to protect the ink formulation, it would be desirable to have a markable ink formulation less prone to discoloration during over lamination by melt extrusion.